1. Field of the Invention
The present invention relates to an electric circuit for supplying a sharply rising, large current with a low loss when a current is commutated from a first circuit to a second circuit, a pulse power source for generating a high-voltage pulse from the second circuit using the electric circuit, and a pulse power source for successively outputting pulses of positive polarity and pulses of negative polarity.
2. Description of the Related Art
In recent years, technologies for deodorization, sterilization, film growth, and toxic gas decomposition based on a plasma produced by discharging high-voltage pulses have been put to practical use (see, for example, Japanese Patent No. 2649340 and Applied Physics, Vol. 61, No. 10, 1992, pages 1039 through 1043, “Deposition of a—Si:H based film by high voltage pulse discharge CVD”). It has been recognized that it is necessary to supply high-voltage pulses of very short pulse duration for efficient plasma processing (see, for example, IEEE TRANSACTION ON PLASMIC SCIENCE, Vol. 28, No. 2, April 2000, pages 434 through 442, “Improvement of NOx Removal Efficiency Using Short-Width Pulsed Power”).
A pulse power source as disclosed in Japanese Laid-Open Patent Publication No. 2002-359979 has been proposed. As shown in FIG. 29 of the accompanying drawings, the proposed pulse power source 100 comprises an extremely simple circuit having a DC power supply 102, an inductor 104, a first semiconductor switch 106, and a second semiconductor switch 108 which are connected in series across the DC power supply 102, and a diode 110 having a cathode connected to an end of the inductor 104 whose other end is connected to the anode terminal of the first semiconductor switch 106, and an anode connected to the gate terminal of the first semiconductor switch 106.
When the second semiconductor switch 108 is turned on, the first semiconductor switch 106 becomes conductive, applying the voltage from the DC power supply 102 to the inductor 104 to store induced energy in the inductor 104. When the second semiconductor switch 108 is thereafter turned off, the first semiconductor switch 106 is quickly turned off, developing a sharply rising extremely narrow high-voltage pulse Po across the inductor 104. Therefore, the high-voltage pulse Po appears between output terminals 112, 114 of the inductor 104.
The pulse power source 100 is of a simple circuit arrangement which is capable of supplying the high-voltage pulse Po which has a sharply rising time and an extremely short pulse duration, without using a plurality of semiconductor switches to which a high voltage is applied.
It is hoped that the pulse power source shown in FIG. 29 will be able to supply the secondary side with a sharply rising, large current with a low loss.
With the pulse power source shown in FIG. 29, electrodes are connected respectively to the output terminals and are accommodated in a reactor for causing a plasma reaction. In a period for storing electromagnetic energy in the inductor (charging period), a voltage induced in the inductor (induced voltage) is applied to a load connected to the secondary side (e.g., electrodes in the reactor), tending to bring about an arc discharge in the reactor. When an arc discharge is produced, an overcurrent flows in a primary circuit of the inductor (main circuit), tending to adversely affect the various semiconductor switches.
A pulse power source used to generate a plasma by changing an electric field to accelerate electrons, employs a process of successively outputting pulses of opposite polarities, i.e., pulses of positive polarity and pulses of negative polarity, in order to generate a high potential difference with a low voltage.
As shown in FIG. 30 of the accompanying drawings, a conventional pulse power source 200 according to the above process has a DC power supply 202, a first switch 204 and a second switch 206 that are connected in series with each other across the DC power supply 202, a third switch 208 and a fourth switch 210 that are connected in series with each other across the DC power supply 202, and a transformer 214 having a primary winding 212 connected between a contact a1 between the first switch 204 and the second switch 206 and a contact a2 between the third switch 208 and the fourth switch 210. The conventional pulse power source 200 is of a bridge configuration. An output voltage Vout is produced across a secondary winding 216 of the transformer 214.
When the second switch 206 and the third switch 208 are turned on, a negative voltage is output across the secondary winding 216 as shown in FIG. 31 of the accompanying drawings. When the second switch 206 and the third switch 208 are turned off after a certain period of time, a negative pulse 218 is generated. When the first switch 204 and the fourth switch 210 are turned on, a positive voltage is output across the secondary winding 216. When the first switch 204 and the fourth switch 210 are turned off after a certain period of time, a negative pulse 220 is generated.
However, the conventional pulse power source 200 is disadvantageous in that the four switches 204, 206, 208, 210 need to be used to form a bridge, resulting in a large number of parts required. Therefore, the conventional pulse power source 200 is large in size and high in cost.
Furthermore, as described above, it is necessary to supply a sharply rising, large current to flow in the secondary side with a low loss and also not to apply an induced voltage to the electrodes in the reactor during a charging period.
Furthermore, depending on appropriate conditions of the application, it is necessary to be able to change the pulse duration of positive pulses and the pulse duration of negative pulses independently of each other.